Quartz optical waveguide and method for producing the same

ABSTRACT

A quartz optical waveguide comprising a substrate, a ridge-form core part formed on said substrate and a part which surrounds said core part and has a lower refractive index than that of said core part, wherein a refractive index changes continuously at an interface between said core part and said part having the lower refractive index, which has a decreased transmission loss and a connection loss when connected with an optical fiber.

This is a division of application Ser. No. 07/762,306, filed Sep. 20,1991, now U.S. Pat. No. 5,179,614.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a quartz optical waveguide which canreduce transmission loss of guided light and a method for producing thesame.

2. Description of the Related Art

Among optical waveguides, a quartz optical waveguide comprising quartzglass attracts attentions since it has a low light transmission loss andcan be connected with a quartz optical fiber with a low connection loss.

In general, such quartz optical waveguide is produced by a combinedmethod of glass film formation by a flame hydrolysis deposition (FHD)and fine processing of the formed glass film by reactive ion etching(RIE) (cf. Masao Kawachi, "Quartz Optical Waveguides and TheirApplication in Integrated Optical Elements", OPTICS, 18 (12), December1989, 681-686).

The above method for producing a quartz optical waveguide will beexplained by making reference to FIG. 1.

As shown in FIG. 1A, a glass-forming raw material such as SiCl₄, TiCl₄and the like are supplied to a burner 2 together with a fuel gas (e.g.hydrogen gas, oxygen gas, etc.) and hydrolyzed and oxidized in anoxyhydrogen flame 2 to form fine particles 3 (soot) of glass. The glasssoot is then deposited on a substrate 4 such as a silicon wafer tosuccessively form films of glass soot 5a and 5b which have differentcompositions from each other. The deposited glass films on the substrate4 are vitrified by heating them at a high temperature to obtain abuffering layer 6a and a core layer 6b as shown in FIG. 1B.

The above method is FHD.

Then, by RIE, unnecessary parts of the core layer 6b are removed toremain a ridge-form core part 6c as shown in FIG. 1C. Again, by FHD, acladding layer 6d is formed to surround the core part 6c to form anembedded type quartz optical waveguide 7 as shown in FIG. 1D.

Though the light transmission loss through the quartz optical waveguidehas been reduced to about 0.1 dB/cm, its further decreased is desiredsince a light transmission loss through a quartz optical fiber has beenreduced to 1 dB/km.

As quality of a device comprising optical waveguide has been muchimproved, a longer waveguide length is required. Then, it is animportant object to further decrease the light transmission loss of thequartz optical waveguide.

The light transmission loss through the optical waveguide may beattributed to light scattering caused by irregular structures such asirregularities at an interface between a core part and a low refractiveindex part which surrounds the core part such as a cladding part or abuffering part.

The reason why the optical fiber has a much lower light transmissionloss may be that, in the fabrication of the optical fiber, since a glasspreform is once produced and it is drawn to a fiber having a diameterof, for example 125 μm, irregularities at an interface between the corepart and the cladding or buffering part are smoothened during drawing sothat such irregularities have no substantial influence on the lighttransmission loss in the fabricated optical fiber.

On the other hand, when the quartz optical wave-guide is produced by themethod which has been explained by making reference to FIG. 1, theirregular structures which are formed during the production of theoptical waveguide remain. In particular, when the unnecessary parts ofthe core layer are removed in the RIE step, it is very difficult to makethe side walls of the core sufficiently smooth and to make the width ofthe core uniform in the longitudinal direction.

FIG. 2 schematically shows refractive index profiles in the commonoptical waveguide, and the refractive index discontinuously changes atinterfaces between the core part and the cladding layer and between thecore part and the buffering layer, that is, the refractive index has aso-called step profile. Therefore, when the optical waveguide isconnected with an optical element having a different mode field such asthe optical fiber, the connection loss increases.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a quartz opticalwaveguide which has a reduced light transmission loss.

Another object of the present invention is to provide a quartz opticalwaveguide which can reduce a connection loss when connected with anoptical element having a different mode field from that of the opticalwaveguide.

A further object of the present invention is to provide a method forproducing a quartz optical waveguide of the present invention.

According to a first aspect of the present invention, there is provideda quartz optical waveguide comprising a substrate, a ridge-form corepart formed on said substrate and a part which surrounds said core partand has a lower refractive index than that of said core part, wherein arefractive index changes continuously at an interface between said corepart and said part having the lower refractive index.

According to a second aspect of the present invention, there is provideda method for producing a quartz optical waveguide which comprises stepsof forming a ridge form core part on a substrate, forming a part whichhas a lower refractive index than that of said core part to surroundsaid core part to form a quartz optical waveguide, and heating saidquartz optical waveguide so as to diffuse a component which changes arefractive index in said quartz optical waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D schematically shows the steps of the flame hydrolysisdeposition method for producing a quartz optical waveguide,

FIG. 2 shows a cross section and refractive index profiles of aconventional quartz optical waveguide,

FIG. 3 shows a perspective view of an example of a quartz opticalwaveguide according to the present invention,

FIG. 4 shows a cross section and refractive index profiles of a quartzoptical waveguide after heating,

FIG. 5 shows steps of the production of a quartz optical waveguideaccording to the present invention,

FIG. 6 shows a refractive index profile after forming a buffering layerand a core layer on a substrate in a direction perpendicular to a layerplane,

FIG. 7 shows refractive index profiles of a quartz optical waveguideproduced in Example 1 before and after heating,

FIG. 8 shows refractive index profiles of a quartz optical waveguideproduced in Example 2 before and after heating,

FIG. 9A shows a quartz optical waveguide and an optical fiber which areabutted with each other before heating the optical waveguide,

FIG. 9B shows a cross section of a furnace for heating the opticalwaveguide in Example 3,

FIG. 9C is a temperature profile in the furnace of FIG. 9B,

FIG. 9D is a distribution of a mode field diameter of the opticalwaveguide after heating in Example 3, and

FIG. 9E shows the quartz optical waveguide and the optical fiber whichare abutted with each other after heating in Example 3.

DETAILED DESCRIPTION OF THE DRAWINGS

As shown in FIG. 3, one example of the quartz optical waveguide 10 ofthe present invention comprises a substrate 4, a ridge-form core part 6cand parts which surround the core part and have a lower refractive indexthan that of the core part, namely, a cladding layer 6d and a bufferinglayer 6a. This optical waveguide has refractive index profiles each ofwhich continuously and smoothly changes in the form of a Gaussiandistribution or an errorfunction distribution as shown in FIG. 4.

If the quartz optical waveguide having the refractive index profiles ofFIG. 4 is to be produced by the conventional method, it is possible torealize such refractive index profile in a direction perpendicular tothe plane (the B-B' direction) by gradually changing a composition ofthe glass-forming raw material as time passes, but it is impossible torealize such refractive index profile in a direction parallel with theplane (the A-A' direction) since the unnecessary parts are cut away fromthe core layer in the perpendicular direction and then the claddinglayer is formed around the remaining core part.

According to the present invention, the glass soot films 5a and 5b areformed on the substrate 4, they are vitrified to from the buffer layer6a and the core layer 6b respectively, the unnecessary parts of the corelayer 6b are removed by RIE to form the ridge core part 6c, and then thecladding layer 6d is formed to surround the core part 6c by the samemethod as the conventional method of FIG. 1. Thereafter, the producedquartz optical waveguide is heated to diffuse a component which changesthe refractive index in the quartz optical waveguide to obtain thequartz optical waveguide 10 having the refractive index profiles asshown in FIG. 4.

Examples of the component which changes the refractive index arerefractive index modifiers such as GeO₂ or F. In addition, a compoundwhich can be easily diffused in the the quartz optical waveguide such asTiO₂, P₂ O₅, B₂ O₃ and the like may be used. GeO₂ and F are preferredsince they have large diffusion rates in the quartz glass and no adverseinfluences on the light transmission loss.

When GeO₂ is used as the refractive index modifier, GeCl₄ is added tothe glass-forming raw material comprising SiCl₄ during the glass sootsynthesis step shown in FIG. 1A. Since synthesized GeO₂ quicklydissipates from the glass soot particles in the heating step, it isremoved during the vitrification step and a part of the dissipated GeO₂is redeposited on the buffering glass layer so that the buffering layerand the core layer may not be clearly distinguished. To overcome suchdefect, the buffering layer forming step and the core layer forming stepshould be separated. That is, after the buffer layer is formed andvitrified, the core layer containing GeO₂ is formed and vitrified.

When F is used as the refractive index modifier, since F lowers therefractive index of the quartz glass, it should be added to the lowerrefractive index parts such as the buffering layer and the claddinglayer. The fluorine is added to the glass by heating the glass sootlayer in an atmosphere containing fluorine to diffuse F in the glasssoot layer rather than a fluorine-containing compound is added to theglass-forming raw material. The fluorine may be added to the glass priorto the vitrification step at a temperature slightly lower than avitrification temperature or during the vitrification step.

In the present invention, it is possible to continuously change the modefield shape of the optical waveguide in the longitudinal direction byheating the optical waveguide part by part rather than heating the wholeoptical waveguide.

For example, to produce a core having a longer optical path in a limitedspace of a small substrate, it is necessary to bend the opticalwaveguide at a small curvature. In such case, to minimize a flexuralloss, it is necessary to make the mode field of the optical waveguide assmall as possible.

The optical waveguide is partly heated near its end which is connectedto the optical fiber to diffuse the component which changes therefractive index so that a mode field diameter of such end part isenlarged to make the mode field diameter identical with that of theoptical fiber, and the mode field diameter of the remaining part of theoptical waveguide is gradually decreased towards the other end byreducing the degree of the diffusion of the component which changes therefractive index.

The partial heating of the optical waveguide can be easily done by usinga temperature profile in an electric furnace as explained in the belowdescribed examples, though a CO₂ laser, a small-size electric furnace ora microtorch may be used.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will be illustrated by following Examples.

EXAMPLE 1

The production steps will be explained by making reference to FIG. 5.

On a silicon substrate 14, a buffering glass soot layer 15a consistingof SiO₂ --P₂ O₅ --B₂ O₃ (contents of P₂ O₅ and B₂ O₃ : 0.4 and 5% bymole) was formed by a flame deposition method. P₂ O₅ and B₂ O₃ wereadded to the glass by adding suitable amounts of POCl₃ and BCl₃ to SiCl₄which was supplied to the oxyhydrogen burner. The glass soot layer 15awas heated at 1250° C. for 2 hours in an atmosphere of 90% of helium and10% of oxygen to form a buffering layer 16a having a thickness of about20 μm.

On the buffering layer 16a, a core glass soot layer 15a consisting ofGeO₂ --SiO₂ --P₂ O₅ --B₂ O₃ (contents of GeO₂, P₂ O₅ and B₂ O₃ : 0.6 and5.5% by mole) was formed by the flame deposition method. GeO₂, P₂ O₅ andB₂ O₃ were added to the glass by adding suitable amounts of GeCl₄, POCl₃and BCl₃ to SiCl₄ which was supplied to the oxyhydrogen burner. The coreglass soot layer 15a was vitrified in the same manner as above to form acore layer 16b having a thickness of about 8 μm. A difference of therefractive index between the buffering layer 16a and the core layer 16bwas 0.4%, and a refractive index profile in the thickness direction wasas shown in FIG. 6, in which a relative distance was "0" (zero) at theinterface between the core and the buffering layers.

Then, the core layer 16b was patterned by a lithographic method, and aridge-form core part 16c having a cross section of 7 μm×7 μm was formedby RIE. During RIE, a top layer of the core part having a varyingrefractive index was etched by a thickness of about 1 (one) μm from itssurface. Accordingly, the refractive index in the core part 16c was 0.4%higher than that of the buffering layer 16a.

Finally, a cladding layer 16d having the same composition as thebuffering layer was formed by the same FHD method and vitrified toobtain an embedded type quartz optical waveguide 17 comprising theridge-form core part 16c having a cross section of 7 μm×7 μm and thelower refractive index parts 18 consisting of the buffering layer 16aand the cladding part 6d.

The produced quartz optical waveguide had a light transmission loss of0.15 dB/cm at a wavelength of 1.3 μm.

After heating the produced optical waveguide at 1200° C. for 12 hours inan inert gas atmosphere, the core diameter was enlarged to about 9 μm asshown in FIG. 7. The optical waveguide in this state had an improvedlight transmission loss of 0.09 dB/cm at the wavelength of 1.3 μm.

EXAMPLE 2

After forming the buffering glass soot layer 15a on the siliconsubstrate 14 in the same manner as in Example 1, the glass soot layerwas vitrified by heating it at 1200° C. for 3 hours in an atmosphere of80% of helium, 10% of SiF₄ and 10% of oxygen.

Then, a core glass soot layer 15b having the same composition as theabove buffering glass soot layer 15a (before F addition) and heated at1250° C. for 2 hours in an atmosphere of 90% of helium and 10% of oxygento obtain a core layer 16b having a thickness of about 7 μm.

The formed buffering layer 16a had a thickness of 20 μm, and thefluorine was added to the buffering layer so that the refractive indexof the buffering layer was 0.4% lower than that of the core layer.

Then, the core layer 16b was patterned by a lithographic method, and aridge-form core part 16c having a cross section of 7 μm×7 μm was formedby RIE.

Finally, a cladding layer 16d having the same composition as thebuffering layer 16a was formed by the same flame deposition method andvitrified to obtain an embedded type quartz optical waveguide 17comprising the ridge-form core part 16c having a cross section of 7 μm×7μm and the lower refractive index parts 18 consisting of the bufferinglayer 16a and the cladding part 6d.

The produced quartz optical waveguide had a light transmission loss of0.2 dB/cm at a wavelength of 1.3 μm.

After heating the produced optical waveguide at 1200° C. for 4 days inan inert gas atmosphere, the core diameter was enlarged to about 8 μm asshown in FIG. 8. The optical waveguide in this state had an improvedlight transmission loss of 0.07 dB/cm at the wavelength of 1.3 μm.

EXAMPLE 3

In the same manner as in Example 1, a quartz optical waveguide 17 havingthe same sizes as those of the optical waveguide shown in FIG. 3 wasproduced. Then, as shown in FIG. 9A, one end surface of the opticalwaveguide 17 was abutted with an end surface of a single mode opticalfiber 20 having a refractive index difference of 0.3% between a core anda cladding and a mode field diameter of 9.5 μm, and a connection losswas measured to be 0.2 dB at a wavelength of 1.3 μm.

Separately, the optical waveguide 17 was inserted in an electric furnace22 having a pair of heaters 21, 21 as shown in FIG. 9B and placed at acenter part of the furnace 22 having the maximum temperature. Then, theoptical waveguide was heated with a maximum temperature of 1200° C. for12 hours. A temperature profile in the furnace 22 is shown in FIG. 6C.

The heated optical waveguide had a continuously changing mode fielddiameter in the longitudinal direction as shown in FIG. 9D.

Then, the heated optical waveguide was cut into two parts at the centerto form optical waveguides 20A. The heated end surface 23 of one of theoptical waveguides 20A was abutted with the same single mode opticalfiber as used in the above. A connection loss was decreased to lowerthan 0.1 dB at a wavelength of 1.2 μm.

What is claimed is:
 1. A quartz optical waveguide comprising:asubstrate; a buffering layer formed on said substrate; a ridge-form corelayer formed on said buffering layer forming a first interface betweensaid buffering layer and said ridge-form layer; and a cladding layerformed on said ridge-form core layer and said buffering layer forming asecond interface between said cladding layer and said ridge-form layer,said cladding layer and buffering layer each having a lower refractiveindex than that of said ridge-form core layer, said first and secondinterfaces having a refractive index which varies continuously in adirection perpendicular to a longitudinal direction of said waveguideand away from a center of said ridge-form core layer.
 2. The quartzoptical waveguide according to claim 1, wherein said ridge-form corelayer is made of quartz containing GeO₂ and said buffering layer andcladding layer are made of pure quartz.
 3. The quartz optical waveguideaccording to claim 1, wherein said ridge-form core layer is made of purequartz and said buffering layer and cladding layer are made of quartzcontaining fluorine.